The present invention relates to the revolutionary wood flake molding technology invented by wood scientists at Michigan Technological University during the latter part of the 1970s.
Wood flake molding, also referred to as wood strand molding, is a technique for molding three-dimensionally configured objects out of binder coated wood flakes having an average length of about 1xc2xc to about 6 inches, preferably about 2 to about 3 inches; an average thickness of about 0.005 to about 0.075 inches, preferably about 0.015 to about 0.025 inches; and an average width of 3 inches or less, most typically 0.25 to 1.0 inches, and never greater than the average length of the flakes. These flakes are sometimes referred to in the art as a xe2x80x9cwood strands.xe2x80x9d This technology is not to be confused with oriented strand board technology (see e.g., U.S. Pat. No. 3,164,511 to Elmendorf) wherein binder coated flakes or strands of wood are pressed into planar objects. In wood flake or wood strand molding, the flakes are molded into three-dimensional, i.e., non-planar, configurations.
In wood flake molding, flakes of wood having the dimensions outlined above are coated with MDI or similar binder and deposited onto a metal tray having one open side, in a loosely felted mat, to a thickness eight or nine times the desired thickness of the final part. The loosely felted mat is then covered with another metal tray, and the covered metal tray is used to carry the mat to a mold. (The terms mold and die, as well as mold die, are sometimes used interchangeably herein, reflecting the fact that xe2x80x9cdiesxe2x80x9d are usually associated with stamping, and molds are associated with plastic molding, and molding of wood strands does not fit into either category.) The top metal tray is removed, and the bottom metal tray is then slid out from underneath the mat, to leave the loosely felted mat in position on the bottom half of the mold. The top half of the mold is then used to press the mat into the bottom half of the mold at a pressure of approximately 600 psi, and at an elevated temperature, to xe2x80x9csetxe2x80x9d (polymerize) the MDI binder, and to compress and adhere the compressed wood flakes into a final three-dimensional molded part. The excess perimeter of the loosely felted mat, that is, the portion extending beyond the mold cavity perimeter, is pinched off where the part defining the perimeter of the upper mold engages the part defining perimeter of the lower mold cavity. This is sometimes referred to as the pinch trim edge.
U.S. Pat. No. 4,440,708 and U.S. Pat. No. 4,469,216 disclose this technology. The drawings in U.S. Pat. No. 4,469,216 best illustrate the manner in which the, wood flakes are deposited to form a loosely felted mat, though the metal trays are not shown. By loosely felted, it is meant that the wood flakes are simply lying one on top of the other in overlapping and interleaving fashion, without being bound together in any way. The binder coating is quite dry to the touch, such that there is no stickiness or adherence which hold them together in the loosely felted mat. The drawings of U.S. Pat. No. 4,440,708 best illustrate the manner in which a loosely felted mat is compressed by the mold halves into a three-dimensionally configured article (see FIGS. 2-7, for example).
This is a very unusual molding process as compared to a molding process one typically thinks of, in which some type of molten, semi-molten or other liquid material flows into and around mold parts. Wood flakes are not molten, are not contained in any type of molten or liquid carrier, and do not xe2x80x9cflowxe2x80x9d in any ordinary sense of the word. Hence, those of ordinary skill in the art do not equate wood flake or wood strand molding with conventional molding techniques.
One limitation heretofore associated with this technology has been the need to design parts with only about a three degree (30xc2x0) draft angle, i.e. the angle of the mold cavity edge with respect to the vertical, where the edge approaches the pinch trim edge. It has been thought that larger draft angles would lead to blistering, over densification, or springback of the wood flakes from the edge, to cause loss of structural integrity. This has resulted in the inability to use wood strand molding technology to make parts whose configuration would require a greater draft angle to the pinch trim edge.
In the present invention, it has been surprisingly discovered that molded wood strand products can be produced having minimal blistering, minimal springback, and minimal over densification to the pinch trimmed edges of the finished product utilizing a mold having a draft angle of ten degrees (10xc2x0) or greater for at least a portion of its edge which approaches the pinch trim edge.